The present invention relates to a silenced blowing nozzle for emitting a gas medium, in particular air, under high overpressure.
For many years within the engineering industry, blowing nozzles of so-called xe2x80x9csilent typexe2x80x9d have been used, i.e. blowing nozzles which for a given blowing force are considerably quieter than corresponding standard blowing nozzles. Belonging to this group of blowing nozzles are tapered slot nozzles of type Silvent(copyright) 511 and 512, cupped hole nozzles of type Silvent(copyright) 208 and 209 and blowing nozzles with flat ends, type Silvent (copyright) 701-720. These blowing nozzles are used for low and moderate blowing forces and blowing distances. So-called xe2x80x9clarge blowersxe2x80x9d are used where large blowing forces are required at long distances. Belonging to this group are aggregates consisting of a larger number of co-operating hole nozzles, which belong to the Silvent(copyright) 1100- and 1200-series of the same applicant. These tools are used for instance for applications in steel plants, paper mills, and foundries for cleaning, cooling, drying etc.
However in certain cases within the pulp and paper industry, blowing nozzles with even higher air flows are used, which generate extremely high noise levels due to the expansion of the air stream after it has left the nozzle. The operator can be subject to a level of approx. 115 dB(A), and for other personnel in the vicinity of the discharge, it is not unusual with values in the range 100-110 dB(A). As the nozzle is often required for sudden interruptions in production at the factory, e.g. when a paper web goes out of line, high requirements are placed on the personnel for immediate action. Many times one simply does not have time to put on hearing protection, which in unfortunate cases can imply permanent hearing damage after only a few seconds of exposure time.
The powerful air nozzles used within the pulp and paper industry can be said to have two areas of application. In one case the air is used as a bearing surface for the paper web in connection with start-up of the paper machine, xe2x80x9cpulls the leading endxe2x80x9d. In this case the air must act as a guide, helping to steer the paper web between rollers in the paper machine. In this case it is suitable that the flow be moderately large and that it be distributed over a large area. The other case is when the paper web has broken and a quickly growing amount of paper must be blown immediately away from the machine at the same time as the leading end must be steered into the correct position. For cleaning, a very strong, concentrated air stream is required, which tends to tear apart the web even at large distances from the nozzle itself; the distance can reach up to 10 m! Other devices for managing the said tasks are not available within known technology. Certain limited tasks can be managed by blowing nozzles with fixed installation, but in all essential work the hand-regulated blowing nozzle, which generates extremely high noise levels, is necessary for giving the required flexibility in use.
The object of the present invention is to offer an efficient blowing nozzle with which a significantly higher and/or quieter blowing force can be achieved for a given frontal area than with corresponding known nozzles.
The invention has been developed especially to solve the above-mentioned problems and to meet needs within the pulp and paper industry, and hereby aims to offer a blowing nozzle which can generate very large blowing forces at significantly lower noise levels than for comparable conventional nozzles. Other areas where these nozzles can be used are e.g. steel plants, foundries etc. The principles of the invention can, however, also be applied to nozzles for small or moderate blowing forces, where the nozzle according to the invention can replace conventional or silenced blowing nozzles employed within the engineering industry.
To achieve the desired blowing force, the nozzle according to the invention comprises at least one first discharge opening in a central part of the nozzle, where the first discharge opening is diverging, suitably formed as a Laval nozzle, to give the discharging gas, normally air, supersonic velocity at the pressure prevailing most immediately behind the discharge. For a correctly formed Laval nozzle, the pressure of the air/gas is converted completely to kinetic energy, which implies that the gas stream does not expand sideways after it has left the nozzle, as is the case for conventional nozzles, where the expansion creates intense noise. A powerful noise occurs nevertheless when gas flows with supersonic velocity out of a correctly dimensioned Laval nozzle. This is assumed caused by violent turbulence arising in the boundary zone between the gas/air stream which rushes forward with a very high velocity, and the surrounding air. The invention aims to solve this problem. According to the invention, the vortex formation in a gas exiting with supersonic velocity in a core stream near said first discharge opening, and therewith the generation of high frequency sound within the audible region, is suppressed in that the core stream is surrounded by a gas flow aimed in the direction of the core stream, which prevents or significantly reduces vortex formation of the core stream near said discharge opening, by which the initially mainly laminar character of the core stream is preserved to a large degree at least within a critical region near the discharge, where the velocity of the core stream is greatest.
The invention is thus based on the interaction of two principles:
1. The core stream is formed such that the working capacity thereof becomes maximum by said core stream emitted through an expanding (diverging) exit (discharge) opening which is formed suchxe2x80x94preferably in the form of a Laval nozzlexe2x80x94that the internal energy of the gas is almost completely transformed into velocity under the influence of the pressure prevailing immediately behind the exit opening. For the dimensional ranges specific to the invention, the velocity in the discharge section of the nozzle lies far above sonic velocity.
2. The formation of turbulence around the rapidly gushing core stream is decreased by said core stream being surrounded by a protective gas flow aimed in the direction of said core stream. The velocity of the surrounding flow shall be lower than that of the core stream. The protective gas flow is released by a larger number of smaller exit (discharge) openings situated around the core streamxe2x80x94this is to suppress vortex formation due to the interaction with surrounding air and therewith also to suppress the generation of sound within the audible region. The most favourable condition is reached if the velocity of the protective gas flow decreases gradually with increased distance from the centre line.
Acoustically, the combination of these principles implies that the sound generation becomes relatively low in that the turbulence of the core stream is suppressed in a region downstream of the discharge orifice within which powerful generation of high frequency sound within the audible region otherwise takes place.
Mechanically, the combination implies a nozzle with a very high degree of efficiency, as the surrounding gas flow causes insignificant slowing down of the velocity of the core stream in the critical region after the orifice by the surrounding stationary air, as most of the mechanical work in accelerating the stationary air in the direction of the core stream is carried out by the surrounding gas flow.
The outstanding feature of the invention is thus that the blowing nozzle in a central part thereof has at least one first exit (discharge) opening formed to generate a core stream of gas with supersonic velocity and that the central part is surrounded by a more peripheral part containing a number of second discharge openings at a distance from each other and from the said first discharge opening(s), which second discharge openings are formed to generate a gas flow with lower velocity than that of the core stream, preferably a velocity equal to sonic velocity, which gas flow surrounds and has the same direction as said core stream.
Said first discharge opening can have a diameter at the most narrow section of up to between 2 and 20 mm, preferably to between 4 and 10 mm, preferably maximum 7 mm and most preferably up to between 5 and 6 mm.
The second discharge openings, especially when these are arranged in the periphery of the nozzle, can be advantageously formed as thin slit openings which extend radially across the projected end area of the nozzle, perpendicular to the longitudinal axis thereof. To form a blowing nozzle with such slit-formed, radially oriented discharge openings in the periphery of the nozzle is known per se through e.g. EP 0 224 555 and the principle is practised in the 700-series of Silvent AB, see above, but has according to the invention at least two purposes in the nozzle. Firstly, the peripheral discharge openings act so that the blowing force reaches a high degree of efficiency even at large distances, secondly the gas stream flowing out through the peripheral openings and surrounding the central gas stream which flows out with supersonic velocity, muffles the otherwise very powerful sound which forms by interaction between the central gas stream with supersonic velocity and the surrounding air, by suppressing the turbulence of the core stream in a critical region. Thus the noise has, on trials done with blowing nozzles according to the invention and compared with a conventional nozzle in the paper industry, at a working pressure of 500 kPa, been reduced from 115 dB(A) for the conventional nozzle to 100 dB(A) for the new nozzle and this with maintained or amplified blowing force. This extraordinarily effective reduction in noise can be utilized for significantly improving the working conditions at existing compressed air equipment and/or for making new equipment significantly less expensive.
Starting with the theory that a good reduction in noise is favoured by a successively decreasing difference in discharge velocity from the central core stream to surrounding air, one can also consider that further discharge openingsxe2x80x94tertiary, fourth, etcxe2x80x94be arranged between said first and second discharge openings, by which these interjacent discharge openings may be formed so that the gas streaming out of these openings also reaches supersonic velocity, although not as high as the supersonic velocity of the central stream. With this developmental embodiment, the tertiary discharge openings arranged around the first discharge opening(s) should thus be shaped to give an air velocity only somewhat lower than the velocity in the core stream, while, if even further discharge openings, here called fourth discharge openings, are arranged between said tertiary and second discharge openings, the said fourth discharge openings are formed such that they give an air velocity which is somewhat higher than sonic velocity, although lower than the velocity from the tertiary discharge openings, and so on.
Said possibly occurring tertiary, fourth etc discharge openings can also be formed as Laval nozzles to make supersonic velocity possible, but in order not to give the maximum possible supersonic velocity, some form of pressure reducer, e.g. restriction flange or similar contraction, should be arranged in the inlet lines.
As high sound frequencies are easier to muffle than low ones, it is acoustically advantageous to replace one large discharge outlet with several small ones. This principle has been utilized for nozzles which work at discharge velocities equal to sonic velocity, but can also be applied to Laval nozzles. For a circular discharge outlet, maximum sound generation occurs at a frequency fmax which is proportional to the diameter of the outlet d and the discharge velocity w. It can therefore be advantageous to use several Laval nozzles in a central part of the blowing nozzle instead of one larger nozzle. An embodiment of the invention is characterized by such an arrangement.
The energy content of the sound generated from the second, peripheral discharge openings should have maximum at a frequency above 20 kHz, that is above the normal upper limit for human hearing. This can be achieved by making the discharge openings as narrow as possible without risk for blocking due to contamination of the compressed air. At the same time, the discharge area and therewith gas flow should be sufficient to suppress said vortex formation to desired degree of significance, which is achieved by a sufficient number of second discharge openings. More exactly, the total discharge area of the second discharge openings should be 1 to 4 times, preferably 1.5 to 3 times as large as the total discharge area of said first discharge opening(s) considered in the most narrow section of the openings, suitably about 2 times as large. With this division, a large blowing force has been achieved at a low sound level.
Generally, it can be further said that the distance between adjacent discharge openings in each concentric group of discharge openings, that is within the central group consisting of several first discharge openings, possibly tertiary and fourth etc, as well as said second discharge openings, should reach 2 to 5 times the equivalent diameter of the openings, which is the square root of the orifice area of the openings, when the openings are slit-formed or otherwise not round.
The outer radius of the nozzle can be 2.5 to 5 times, preferably approx. 3 times the diameter of the most narrow section in the first discharge opening, when this is composed of a single central Laval nozzle. Further, the radial distance between the innermost part of the second discharge openings and the point on the periphery of the first discharge opening(s) in the orifice should amount to at least a third of the radius of the nozzle, where the radius is defined as the distance from the centre to the outer point of the second discharge openings, and where discharge openings are not arranged between said first and second discharge openings.
Further characteristic features and aspects of the invention will be evident from the patent claims as well as from the following description of a number of conceivable embodiments.